In recent years, in application fields of alternating-current motors such as industrial equipment, home appliances and automobiles, a method to control driving of a permanent magnet synchronous motor with an inverter is becoming popular instead of a method to control driving of an induction motor in the related art with an inverter. A permanent magnet synchronous motor is known as a high efficiency motor compared to an induction motor because of the following features. Since magnetic flux is established by a permanent magnet incorporated in a rotor, current corresponding to excitation current is unnecessary. Since current does not flow through the rotor, secondary copper loss is not generated. Torque is effectively obtained by utilizing reluctance torque caused by a difference of magnetic resistance in the rotor in addition to the torque generated by the magnetic flux of the permanent magnet. Accordingly, applying the permanent magnet synchronous motor to an electric vehicle driving system has been studied in recent years.
A general control method of a permanent magnet synchronous motor is to perform current control by a current control system. A current detection value from a current detector disposed at an output side of an inverter is divided into a d-axis component (i.e., a magnetic flux current component) on a rotation coordinate system rotating in synchronization with a rotation phase of a rotor of a motor and a q-axis component (i.e., a torque current component) being perpendicular thereto. Largeness of voltage applied to the motor is adjusted so that d/q-axes current is matched to d/q-axes current command calculated from a torque command, while dividing.
In the case that a permanent magnet synchronous motor (hereinafter, called a motor) is considered to be applied to a driving system of an electric vehicle, a control apparatus of an alternating-current motor is required to be downsized and lightened since equipment is necessary to be mounted to a limited space under vehicle floor. In general, direct-current on the order of 1500 V to 3000 V is inputted to an inverter incorporated in the control apparatus of an alternating-current motor for an electric vehicle. Therefore, a high voltage resistant switching element being resistant to the order of 3300 V to 6500 V is used. Here, both of switching loss and conduction loss are large with such a high voltage resistant switching element. To avoid requiring an excessive switching element cooling apparatus including a cooler and a cooling fan, the acceptable switching frequency is on the order of 1000 Hz at highest. This is a low value on the order of one tenth to one twentieth compared to the frequency for home appliances, industrial inverters and electric vehicles, for example.
It is important for reducing size and weight of the control apparatus of an alternating-current motor to reduce loss generated by the incorporated switching element in order to reduce size and weight of the cooling apparatus thereof. Accordingly, it is necessary to suppress motor current as low as possible by setting the switching frequency as low as possible and applying inverter input voltage to the motor as much as possible.
Meanwhile, the maximum value output frequency of the inverter (i.e., the output frequency of the inverter at the designed maximum speed of an electric vehicle) for an application of an electric vehicle is approximately 400 Hz. For example, in the case that the output frequency of the inverter is at the vicinity of 400 Hz which is the maximum value, the number of pulses included in a half cycle of the inverter output voltage is approximately 1.875 which is obtained from the switching frequency divided by the output frequency of the inverter resulting in being extremely small, provided that the switching frequency of the inverter is approximately 1000 Hz which is the maximum.
When the motor is driven in such a state, the pulse number and the pulse position included respectively in a positive half cycle and a negative half cycle of the inverter output voltage are imbalanced. Then, symmetry between positive and negative of the voltage applied to the motor (i.e., line voltage) is lost, so that noise and vibration is caused due to generation of electric vibration and torque pulsation with the motor.
Accordingly, a so-called synchronous pulse mode such as a synchronous five-pulse mode, a synchronous three-pulse mode and the like to determine switching timing in synchronization with the inverter output voltage is used in an area where the output frequency of the inverter is relatively high. Further, in the case that the maximum voltage is applied to the motor, the motor is operated using a one-pulse mode in which the inverter output voltage is a rectangular wave. In the synchronous mode and the one-pulse mode, the pulse number and the pulse position included in a half cycle of the inverter output voltage is constant and is not varied with time. Therefore, the pulse number and the pulse position are the same between the positive half cycle and the negative half cycle of the inverter output voltage. Accordingly, since the symmetry between positive and negative of the voltage applied to the motor is maintained, electric vibration and torque pulsation are not generated in the motor.
As described above, in order to stably drive an inverter for an electric vehicle, an asynchronous pulse mode is selected, in which a switching frequency is not in synchronization with an output frequency of the inverter (for example, at 1000 Hz constant), in a driving area where the output frequency of the inverter is relatively low. A one-pulse mode, in which the inverter output voltage is a rectangular wave or a synchronous pulse mode, is selected in a driving area where the output frequency of the inverter is relatively high. That is, the motor is driven while the pulse mode is switched in accordance with the output frequency of the inverter.
In the synchronous pulse mode or the one-pulse mode, the number of pulses included in a half cycle of the inverter output voltage is small. Accordingly, in order to ensure control stability, a configuration is adopted in which decreasing current control response of the above-mentioned current control system, stopping calculation of the current control system, and switching to control of adjusting only phases of voltage applied to the motor are possible.    Patent Document 1: Japanese Patent Application Laid-open 2006-081287